The present invention relates to an improvement of a field emission electron gun systems which are used in electron beam applied apparatus such as a scanning electron microscope (SEM), an electron beam exposure device, a size measuring scanning electron microscope, and the like, using an electron beam, and more particularly relates to an improvement of a magnetic immersion field emission electron gun which is used in the electron beam applied apparatus and capable of reducing an aberration of an electrostatic lens by focusing an electron beam by means of a superposition of a magnetic field over an electrostatic lens system.
In the electron beam applied apparatus, an electron gun assumes an important part for generating electron beam. Recently, there becomes to be used an electric field emission type electron gun which has the high luminance in comparison with a conventional hot cathode. A tip of a cathode in the electric field emission type electron gun is formed in an extremely small needle shape (a radius on or less than 1 .mu.m), and an electron is radially emitted from the cathode. Since a large opening angle of the beam needs to obtain a large current, a beam diameter is caused to be extended because of a spherical aberration of an electrostatic lens system provided in the electron gun, thereby reducing a luminance. Accordingly, a magnetic field superposes over the electrostatic lens system including the cathode to focus the electron beam to thus decrease an aberration of the electrostatic lens. An electron gun having such a construction is called as a magnetic immersion field emission electron gun system.
FIG. 1 is a section view showing an example of a conventional magnetic immersion field emission electron gun. The electron gun is a three-electrode field emission type electron gun which comprises a cathode 101, a first anode 102, and a second anode 103. The second anode 103 is connected with a vacuum vessel 104 to be grounded. A power source 105 supplies to the cathode 101 with an acceleration voltage which is negative to the vacuum vessel 104. Furthermore, the cathode 101 is heated by a power source 107. The first anode 102 is connected to a power source 106 to be positive voltage to the cathode 101.
A single-pole magnetic field lens 108 is provided outside the vacuum vessel 104 to superpose a magnetic field over the electron gun. The single-pole magnetic field lens 108 comprises a coil 109 for generating a magnetic flux, and a magnetic substance 110 for forming a magnetic path.
The electrostatic lens system of the conventionally used magnetic immersion field emission electron gun is a two-polar electron gun comprising a cathode and an anode, or a three-polar electron gun comprising a cathode, a first anode and a second anode. Since these electron guns configured above have a little number of polar and short length of the entire electron gun (a distance from the cathode to the anode), the magnetic superposition near the cathode contributes to a reduction of a spherical aberration.
On the other hand, since the necessity happens to independently set an emission current and cross-over position (a focal point) in the electron beam applied apparatus, the necessity starts the use of the electron gun which independently has a leader electrode and a lens electrode, wherein the leader electrode determines an emission current and the lens electrode determines the cross-over position. However, when the electron gun is comprised of a four-polar construction having a cathode, a leader electrode, a lens electrode, and an anode, it is impossible for the conventional magnetic immersion field emission electron gun superposing the magnetic field near the cathode to sufficiently reduce the spherical aberration. Because aberration occurring positions do not concentrate near the cathode on the basis that there are large numbers of the electrodes in a multi-electrode construction over four electrodes and the electron gun has the long length. Furthermore, in order to obtain the high magnetic strength along the entire electrostatic lens system, there becomes large size of a electromagnet or permanent magnet for generating a magnetic field, thereby lacking usefulness.
For example, in "an electron beam gun" disclosed in the official gazette of Japanese patent laid-open number Heisei 2-37651(1990), which comprises a cathode and three-polar portion of a first to third polar, there is provided a permanent magnet generating a magnetic field for reducing a diameter of an electron beam, which is arranged near a passing hole of the electron beam of the second electrode and within a thickness range of the axial direction of the second electrode. As has been immediately understood from viewing FIG. 1 disclosed in this official gazette, the permanent magnet is arranged in a groove of the second electrode which is formed in U-shape in section, and the magnetic field generated by the permanent magnet makes a flux size of the electron beam be slender for improving a spherical aberration of a main lens.